Circuit Arrangement and Method for Striking a Discharge Lamp

ABSTRACT

A circuit arrangement for striking a discharge lamp, comprising: a drive apparatus, having an output adapted to provide a drive signal with a predeterminable frequency; an inverter, which is coupled to the output of the drive apparatus, and having an output adapted to provide a square-wave signal with a predeterminable duty factor; a load circuit, which is coupled to the output of the inverter and has at least one terminal for the discharge lamp, the load circuit comprising a lamp inductor, which is coupled in series between the output of the inverter and the at least one terminal for the discharge lamp; a first control loop with a first reference variable, a first manipulated variable and a first controlled variable, the first control loop having a first time constant; a second control loop with a second reference variable, an auxiliary manipulated variable and a second controlled variable, the second control loop having a second time constant; and a strike detection apparatus, which is adapted to detect striking of the discharge lamp and, after detection of the striking to switch over the first and the second control loops from the striking operation mode to the continuous operation mode. The auxiliary manipulated variable of the second control loop represents the first reference variable of the first control loop, the first time constant being smaller than the second time constant by at least a factor of 10, and the first manipulated variable represents the duty factor of the output signal of the inverter.

TECHNICAL FIELD

The present invention relates to a circuit arrangement for striking adischarge lamp with a drive apparatus, at whose output a drive signalwith a predeterminable frequency can be provided, an inverter, which iscoupled to the output of the drive apparatus and at whose output asquare-wave signal with a predeterminable duty factor can be provided, aload circuit, which is coupled to the output of the inverter and has atleast one terminal for the discharge lamp, a first control loop with afirst reference variable, a first manipulated variable and a firstcontrolled variable, the first control loop having a first timeconstant, and a second control loop with a second reference variable, anauxiliary manipulated variable and a second controlled variable, thesecond control loop having a second time constant The inventionfurthermore relates to a method for striking a discharge lamp using sucha circuit arrangement.

PRIOR ART

High-pressure and low-pressure discharge lamps require high voltages forstriking which are provided by electronic ballasts. As is generallyknown, discharge lamps are used in a wide temperature range, for examplefrom −25° C. to +60° C. One problem in this case is the fact that theinductance of the lamp inductor which is generally arranged in the loadcircuit is temperature-dependent. Thus, the maximum magnetic fluxdensity and therefore the inductance can fluctuate in the mentionedtemperature range by up to 20%. A particular problem is the fact thatthe inductance is reduced as the temperature increases. Thus, the lampinductor enters saturation earlier at higher temperatures. In the caseof the mentioned reduction in the inductance by 20%, the resonantfrequency of the load circuit, whose resonance is utilized for striking,increases by approximately 10%. If, therefore, the resonant frequency isapproached from high frequencies, resonance and therefore the productionof high currents is already achieved much earlier than at lowertemperatures. This entails the risk of destruction of the switches ofthe inverter, which in particular are often realized as MOSFETs.

This problem has been dealt with in the prior art by the saturationlimit of the lamp inductor having been selected as being very high, withthe result that saturation could safely be ruled out even at highambient temperatures. This results in the following undesirabledisadvantages: firstly, an inductor which has large dimensions in termsof its saturation response requires a lot of space since a largerinductor design needs to be used given the same winding losses duringnormal operation. This also enlarges the housing size of the electronicballast. Both measures increase the costs of the electronic ballastconsiderably.

Secondly, given the same inductor design, but with a higher saturationlimit of the inductor, the power loss of the inductor or the electronicballast increases during normal operation. This results in the necessityfor a larger housing design for the electronic ballast in order that thelife is not shortened as a result of the thermal loading.

DESCRIPTION OF THE INVENTION

The present invention is therefore based on the object of developing thecircuit arrangement mentioned at the outset or the method mentioned atthe outset in such a way that it can be realized with a lamp inductorwhich has markedly smaller dimensions than in the prior art. This objectis achieved by a circuit arrangement having the features of patent claim1 and by a method having the features of patent claim 11.

The present invention is first of all based on the knowledge that, inprinciple, actually the actual value U_(act) of the lamp voltage U_(L)should be monitored and increased in small steps up to the intendedmaximum voltage. However, the present invention is based in particularon the knowledge that, as a result of the saturation of the lampinductor, a nonlinearity occurs between the actual value U_(act) of thevoltage U_(L) across the lamp and the actual value I_(act) of thecurrent I_(D) through the inductor at high temperatures. Owing to thisnonlinearity, monitoring of the voltage is on its own insufficient, andinstead both variables need to be monitored separately since it is nolonger possible to draw conclusions about one variable on the basis ofthe other. Since, however, the current I_(D) increases very rapidly onsaturation of the inductor, the determination of the lamp voltage U_(L),which is slow as a result of measurement and correction time constants,is not quick enough in order to disconnect the inverter switches. In oneexemplary embodiment of the invention, this takes place between 200 and400 μs. If the switch-on time t_(on) of the square-wave signal drivingthe switches of the inverter is from 3 to 10 μs, this also demonstratesthat the closed-loop control of the lamp voltage U_(L) with thespecified time constant is too slow. If it is assumed that theclosed-loop control should be so quick that, given an exponential risein the current I_(D) after saturation, the lamp inductor can be switchedoff quickly enough, one arrives at the concept according to theinvention of subjecting the actual value I_(act) of the inductor currentI_(D) to closed-loop control and monitoring. In the exemplaryembodiment, the latter is possible with a time constant of from 100 to200 ns. Closed-loop control of the actual value I_(act) of the inductorcurrent I_(D) on its own does not, however, take into account the factthat at the same time the lamp voltage U_(L) should be increasedstepwise up to a maximum value, which is in the range of the intendedstriking voltage U_(Z). It is therefore necessary on the one hand toincrease the actual value U_(act) of the lamp voltage U_(L) in order toat some point reach the striking voltage U_(Z), and on the other hand atthe same time to subject the actual value I_(act) of the inductorcurrent I_(D) to closed-loop control in order to ensure that, onsaturation of the lamp inductor, no undesirably high current valuesarise which could result in destruction of the switches of the inverter.

The present invention solves this problem in a particularly clever wayby virtue of the fact that the two control loops, i.e. the naturallyslow control loop for the closed-loop control of the lamp voltage andthe quick control loop for the closed-loop control of the inductorcurrent I_(D), are linked to one another. In particular, they are linkedin such a way that the auxiliary manipulated variable of the secondcontrol loop represents the first reference variable of the firstcontrol loop. If the second reference variable corresponds to thesetpoint value of the voltage across the discharge lamp, the secondcontrolled variable corresponds to the actual value of the voltageacross the discharge lamp, the first reference variable corresponds tothe setpoint value of the current through the inductor, and the firstcontrolled variable corresponds to the actual value of the currentthrough the inductor, thus, in other words, the actual value of the lampvoltage is increased stepwise (slow control loop) and the resultant risein the actual value I_(act) of the inductor current (quick control loop)is monitored.

As has been demonstrated in exemplary embodiments realized, reliableclosed-loop control up to from eight to ten times the saturation currentof the lamp inductor is possible with this arrangement.

The present invention therefore makes it possible to generate an inparticular constant striking voltage independently of temperatureinfluences. A further advantage consists in the fact that thisclosed-loop control can be used to generate stable voltagesindependently of the rate of rise of the load circuit. In addition, as aresult of the invention, the components, in particular the lampinductor, the MOSFETs of the inverter and the capacitors used in thecircuit arrangement, are subjected to a far lesser load during strikingthan in the prior art, as a result of which the life of the componentsis extended. In addition, the lamp inductor can be dimensioned so as tohave fewer losses for normal operation. This means that a smallerinductor design can be used, which in turn contributes to a saving onspace and a cost saving. The monitoring of the actual value U_(act) ofthe lamp voltage in accordance with the present invention moreover alsomakes it possible to realize the capacitors with smaller dimensions anda smaller design, as a result of which high voltages can be avoided.

As an alternative to the proposed exemplary embodiment, in which thesecond reference variable corresponded to the setpoint value of thevoltage across the discharge lamp, and the second controlled variablecorresponded to the actual value of the voltage across the dischargelamp, it is naturally readily possible for the setpoint value of thecurrent through the inductor to be used as the second reference variableand for the actual value of the current through the inductor to be usedas the second controlled variable.

Preferably, the second reference variable corresponds to the temporalmean within a predeterminable time period. In a preferred exemplaryembodiment, this time period is between 200 μs and 1 ms.

While the frequency is varied in the circuit arrangements known from theprior art in order to reach the striking voltage, in the circuitarrangement according to the invention, the frequency preferably remainsfixed. In order that a frequency can be fixedly predetermined, measuresfor varying the frequency are no longer required.

The first time constant is preferably from 10 to 1000 ns, preferablyfrom 100 to 200 ns. The second time constant is preferably from 10 to1000 μs, preferably from 200 to 400 μs. If this is compared with theswitch-on time t_(on) of the signal driving the inverter, whichswitch-on time t_(on) is of the order of magnitude of between 1 and 50μs, preferably between 3 and 10 μs, it can be seen that the closed-loopcontrol of the current is much quicker, and the closed-loop control ofthe voltage is much slower. The frequency of the signal driving theswitches of the inverter is preferably between 30 kHz and 100 kHz. Thefirst control loop is thus so quick that it can be switched off quicklyenough in the case of an exponential rise in the current I_(D) throughthe inductor after start of saturation of the lamp inductor, even beforecritical current ranges occur for the switches of the inverter.

As has already been mentioned, the first control loop and the secondcontrol loop each have an interference variable, which primarilyrepresents the ambient temperature.

Preferably, a circuit arrangement according to the invention furthermorehas a strike detection apparatus, which is designed to detect strikingof the discharge lamp and, after detection of the striking, to switchover the first and the second control loop from the striking operationmode to the continuous operation mode.

Further preferred embodiments are given in the dependent claims.

The preferred embodiments described above with reference to the circuitarrangement according to the invention and the advantages of saidembodiments apply correspondingly, where applicable, to the methodaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWING(S)

An exemplary embodiment of a circuit arrangement according to theinvention will now be described in more detail below with reference tothe attached drawings, in which:

FIG. 1 shows a schematic illustration of the coupling between a firstand a second control loop in a first embodiment of a circuit arrangementaccording to the invention;

FIG. 2 shows a schematic illustration of the coupling between a firstand a second control loop in a second embodiment of a circuitarrangement according to the invention;

FIG. 3 shows the signal flowcharts associated with the embodiment inFIG. 1.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a schematic illustration of the coupling between a first,inner control loop R_(j) and a second, outer control loop R_(a) inaccordance with a first exemplary embodiment of a circuit arrangementaccording to the invention. Accordingly, the reference variable of theouter control loop R_(a) is the setpoint value U_(set) of the lampvoltage U_(L) of a discharge lamp (not illustrated). The feedbackvariable is formed by the actual value U_(act) of the lamp voltageU_(L), which at the same time represents the controlled variable of theouter control loop. The difference ΔU between the setpoint value U_(set)and the actual value U_(act) of the lamp voltage U_(L) represents thecontrol error, which is fed to a block 10, which contains an I element(integrator) and a table or a conversion formula, with which a setpointvalue I_(set) of the inductor current I_(D) can be fixed from theintegral via the difference ΔU, for mean value generation. This setpointvalue I_(set) is used as the reference variable of the inner controlloop There, first the difference ΔI from the actual value I_(act) of theinductor current I_(D) is formed again, which is supplied to a block 12in order to produce a change Δt_(on) in the switch-on time and thereforein the duty factor of the signal to be supplied to the inverter 14,which is preferably in the form of a half-bridge circuit with two MOSFETtransistors, via a formula or a reference table there. The inverter 14feeds the load circuit 16 and in the process generates the actual valueI_(act) of the inductor current I_(D). The actual value I_(act) is usedvia a feedback line 18 as a feedback variable of the inner control loopR_(i). The actual value U_(act) of the lamp voltage U_(L) is formed fromthe actual value I_(act) of the inductor current I_(D) at the lamp 20,in particular the circuitry thereof, which comprises a resonantcapacitor. This actual value U_(act) is used as the feedback variable ofthe outer control loop R_(a).

The time constant of the inner control loop R_(i) is between 10 and 1000ns, preferably between 100 and 200 ns. The time constant of the outercontrol loop R_(a) is between 10 and 1000 μs, preferably between 200 and400 μs.

FIG. 2 shows a schematic illustration of a second exemplary embodimentof the invention with the coupling between a first control loop R_(i)and a second control loop R_(a), the reference symbols introduced withreference to FIG. 1 continuing to apply for identical and similarelements and therefore not being described again. In contrast to FIG. 1,however, in this case the actual value I_(act) of the inductor currentI_(D) is used as the controlled variable of the outer control loopR_(a). Correspondingly, the reference variable of the outer control loopis the setpoint value I_(seta) of the inductor current I_(D). Thisvariable is preferably determined at the resonant capacitor of thestriking circuit, which is part of the load circuit.

FIG. 3 shows the signal flowcharts associated with the exemplaryembodiment in FIG. 1. After the start in step 100, the setpoint valueU_(set) is set to the start value U_(setstart) in step 110. In step 120,a check is carried out to ascertain whether U_(set) is smaller than amaximum value U_(max) of the voltage U_(L) across the lamp. This isintended to ensure, in order to avoid damage to the circuit arrangement,that the range in which the lamp generally is struck is not left. IfU_(set) is above U_(max), this results in interruption of the strikingoperation in step 130. If, on the other hand, U_(set) is below U_(max),the difference ΔU is formed from the present actual value U_(act) of thelamp voltage U_(L) and the predetermined setpoint value U_(set) in step140. This difference ΔU is supplied to the block 10 and produces thepresent setpoint value I_(set) for the inductor current I_(D) in step150. In step 160, a check is now carried out to ascertain whether thepresent value for I_(set) is smaller than a maximum current valueI_(max). If this is not the case, the striking operation is interruptedin step 170. If I_(set) is smaller than I_(max), the difference ΔI isformed from the setpoint value I_(set) and the actual value I_(act) ofthe inductor current I_(D) is formed in step 175. In block 12, the valuefor Δt_(on), i.e. the time period, is determined from the difference ΔIin step 180 in order to increase the switch-on duration of the switchesof the inverter. The present time period t_(on) is thereupon calculatedin step 190. Then, a check is carried out in step 200 to ascertainwhether the lamp has been struck. If this is the case, the continuousoperation mode is activated in step 210. If the check in step 200 showsthat the lamp has not yet been struck, U_(set) is increased by apredeterminable increment ΔU_(set) in step 220 and fed back as thepresent U_(set) to the input of step 120. From the driving with thechanged switch-on time t_(on), a new actual value I_(act) of theinductor current I_(D) is formed in step 230 via the load circuit, whichnew actual value I_(act) is fed back in step 170. An actual valueU_(act) of the lamp voltage U_(L) is formed in step 240 from the actualvalue I_(act) of the inductor current I_(D) via the high-pressuredischarge lamp 20.

In the exemplary embodiment illustrated in FIG. 3, the event of amaximum value U_(max) of the lamp voltage U_(L) being reached orexceeded and the event of a maximum value I_(max) of the inductorcurrent I_(D) being reached or exceeded have been specified asinterruption criteria. In addition or as an alternative, it could beprovided that the lamp is operated over a predeterminable time period ata maximum value U_(max) of the lamp voltage U_(L) prior to aninterruption in step 130, and the interruption is only carried out oncea predeterminable period of time has been exceeded.

1. A circuit arrangement for striking a discharge lamp, comprising: adrive apparatus, having an output adapted to provide a drive signal witha predeterminable frequency; an inverter, which is coupled to the outputof the drive apparatus, and having an output adapted to provide asquare-wave signal with a predeterminable duty factor; a load circuit,which is coupled to the output of the inverter and has at least oneterminal for the discharge lamp, the load circuit comprising a lampinductor, which is coupled in series between the output of the inverterand the at least one terminal for the discharge lamp; a first controlloop with a first reference variable, a first manipulated variable and afirst controlled variable, the first control loop having a first timeconstant; a second control loop with a second reference variable, anauxiliary manipulated variable and a second controlled variable, thesecond control loop having a second time constant; and a strikedetection apparatus, which is adapted to detect striking of thedischarge lamp and, after detection of the striking to switch over thefirst and the second control loops from the striking operation mode tothe continuous operation mode; wherein the auxiliary manipulatedvariable of the second control loop represents the first referencevariable of the first control loop, the first time constant beingsmaller than the second time constant by at least a factor of 10, andthe first manipulated variable represents the duty factor of the outputsignal of the inverter (14).
 2. The circuit arrangement as claimed inclaim 1, wherein the first reference variable corresponds to thesetpoint value of the current through the lamp inductor, and the firstcontrolled variable corresponds to the actual value of the currentthrough the lamp inductor.
 3. The circuit arrangement as claimed inclaim 1, wherein the second reference variable corresponds to thesetpoint value of the current through the lamp inductor, and the secondcontrolled variable corresponds to the actual value of the currentthrough the lamp inductor.
 4. The circuit arrangement as claimed claim1, wherein the second reference variable corresponds to the setpointvalue of the voltage across the discharge lamp, and the secondcontrolled variable corresponds to the actual value of the voltageacross the discharge lamp.
 5. The circuit arrangement as claimed inclaim 3, wherein the second reference variable corresponds to thetemporal mean within a predeterminable time period.
 6. The circuitarrangement as claimed in claim 1, wherein the frequency is fixed. 7.The circuit arrangement as claimed in claim 1, wherein the first timeconstant is from 10 to 1000 ns, preferably from 100 to 200 ns.
 8. Thecircuit arrangement as claimed in claim 1, wherein the second timeconstant is from 10 to 1000 μs, preferably from 200 to 400 μs.
 9. Thecircuit arrangement as claimed in claim 1, wherein the first controlloop has a first interference variable, and the second control loop hasa second interference variable, which variables each represent theambient temperature.
 10. (cancel)
 11. A method for striking a dischargelamp using a circuit arrangement with a drive apparatus, which providesa drive signal with a predeterminable frequency at the output of saiddrive apparatus, with an inverter, which is coupled to the output of thedrive apparatus and which provides a square-wave signal with apredeterminable duty factor at its output, with a load circuit, which iscoupled to the output of the inverter and has at least one terminal forthe discharge lamp, the load circuit comprising a lamp inductor, whichis coupled in series between the output of the inverter and the at leastone terminal for the discharge lamp, with a first control loop with afirst reference variable, a first manipulated variable and a firstcontrolled variable, the first control loop having a first timeconstant, with a second control loop with a second reference variable,an auxiliary manipulated variable and a second controlled variable, thesecond control loop having a second time constant; wherein the methodcomprises the steps of: coupling the first and the second control loopin such a way that the auxiliary manipulated variable of the secondcontrol loop represents the first reference variable of the firstcontrol loop; fixing the first time constant such that it is smallerthan the second time constant by at least a factor of 10; and using theduty factor of the output signal of the inverter as the firstmanipulated variable.